The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to equipment and methods for completing subterranean wells; in particular, wells that are subjected to steam- or water-injection treatments.
During completion of a subterranean well, drilling and cementing operations are performed to provide a conduit through which desirable fluids originating within the formation may flow. The cementing operation involves placing a competent cement sheath inside the annular region between the external surface of a tubular body such as well casing, and the borehole wall. The cement sheath supports the casing and provides a hydraulic seal between producing formations. The presence of a hydraulic seal is commonly referred to as zonal isolation.
Well cementing is a difficult operation because it requires several parameters to be considered and controlled. For example, a slurry that is too dense may cause the formation rock to fracture, while a slurry that is too light may allow formation fluids to intrude. While slurry density is a parameter that is relatively easy to control, this is not true of the rheological properties. Such problems, which are inherent to any well-cementing operation, are well known to the skilled person, and solutions generally involve incorporating various additives into the slurry, the selection of which is not always clear and varies from one well to another. Detailed information about well cementing may be found in the following publication: Nelson E B and Guillot D (eds.): Well Cementing, 2nd Edition, Schlumberger (2006), incorporated in its entirety by reference thereto.
When well cementing is successful, and a cement sheath has been formed that provides casing support and zonal isolation once the slurry has set and hardened, it may not be long before the sheath is subjected to mechanical and/or thermal stresses that can lead to deterioration. Cement systems employed in thermal-recovery wells are particularly prone to problems that lead to loss of zonal isolation. One type of thermal-recovery well involves the injection of steam into the wellbore, commonly known as steamflooding. Steamflooding may consist of introducing steam into an injection well and sending the steam through the formation to one or more production wells. Another technique involves cyclic steam injection, during which steam is injected into a single well for a limited period. After the steam-injection period, the well is placed into production. Heating the reservoir reduces the viscosity of oil in the formation, making production more efficient. Steamflood wells are usually less than 915 m (3000 ft) deep, and are frequently deviated (30° to horizontal). The circulating temperatures during primary-cementing operations are often less than 40° C. (104° F.). During injection, the steam temperature may approach about 315° C. (600° F.).
When heat is initially supplied, the temperature rise is normally controlled to prevent undue thermal shock to the casing and cement. Nevertheless, because of thermal expansion, high levels of stress are built up in the pipe and the cement sheath.
A substantial amount of work has been performed for many years to devise cementing techniques that minimize the effects of thermal expansion. Such methods include the placement of thermal packers and the inclusion of a sliding sleeve in the casing string that can move freely in response to thermal stress. Another procedure involves holding the casing in tension during primary cementing to minimize expansion when thermal stress is eventually applied.
In some instances, methods involve applying internal casing pressure after the primary cementing has been performed, and while the cement slurry is setting and hardening. The internal pressure may vary from about 15.9 MPa to 138 MPa (2300 psi to 20,000 psi). This process prestresses the casing, and gives the cement sheath an improved ability to withstand the application of heat during the steam-injection process. Modeling software is used to analyze the anticipated well conditions during steam production, and determine the optimal amount of casing pressurization. All of the above techniques have aimed at maintaining the strongest possible bonding between the cement sheath and the casing.
Expanding cements have also been proposed in an effort to preserve intimate cement/casing and cement/formation bonding during all stages of the well's life. Unfortunately, when the formation is weak (a common occurrence in steamflood wells), expansion may cause the cement sheath to move away from the casing, creating a microannulus. For this reason, expanding cements for thermal recovery wells are generally not recommended. At the other extreme, cement expansion coupled with thermal expansion of casing and a stronger formation can compress the cement sheath, leading to cracking and even pulverization.
Despite the valuable contributions of the prior art, cement-sheath failures and loss of zonal isolation continue to occur. Therefore, there remains a continued need for improved cementing techniques in the realm of steam-injection wells.